1. Field of the Invention
The present invention relates to a dual mode band-pass filter for use in, for example, communications equipment operating in the range of the microwave band to the milliwave band.
2. Description of the Related Art
Conventionally, different types of dual mode band-pass filters have been proposed as a band-pass filter for use in a high frequency band as described in “MINIATURE DUAL MODE MICROSTRIP FILTERS”, J. A. Curtis and S. J. Fiedziuszko, 1991 IEE MTT-S Digest.
FIGS. 48 and 49 are schematic plan views illustrating conventional dual mode band-pass filters, respectively.
In a band-pass filter 200 shown in FIG. 48, a circular conductive film 201 is provided on a dielectric substrate (not shown). Input-output coupling circuits 202 and 203 are coupled to the conductive film 201 so as to define an angle of 90° relative to each other. A tip-open stub 204 is disposed on the conductive film 201 at a position that defines a central angle of 45° relative to the input-output coupling circuit 203. Thereby, two resonance modes with different resonance frequencies are coupled together. Thus, the band-pass filter 200 is configured so as to operate as a dual mode band-pass filter.
In a dual mode band-pass filter 210 shown in FIG. 49, a substantially square conductive film 211 is disposed on a dielectric substrate. Input-output coupling circuits 212 and 213 are connected to the conductive film 211 so as to define an angle of 90° relative to each other. The corner of the conductive film 211 in the position thereof defining an angle of 135° with respect to the input-output coupling circuit 213 is cut away. The resonance frequencies in two resonance modes are made different from each other by the cut portion 211a, so that the two resonance modes having different resonance frequencies are coupled to each other. Thus, the band-pass filter 210 can be operated as a dual mode band-pass filter.
On the other hand, there has been proposed a dual mode band-pass filter that contains a ring-shape conductive film instead of the circular conductive film (Japanese Unexamined Patent Application Publication No. 9-139612 and No. 9-162610). In particular, a dual mode filter is disclosed, in which a ring-shaped ring transmission line is used, input-output coupling circuits are arranged so as to define a central angle of about 90° similarly to the dual mode band-pass filter 200 shown in FIG. 48, and a tip-open stub is provided in a portion of the ring transmission line.
Moreover, Japanese Unexamined Patent Application Publication No. 6-112701 discloses a dual mode band-pass filter that uses a ring transmission line similar to the above-mentioned transmission line. As shown in FIG. 50, in the dual mode filter 221, a ring resonator includes a ring conductive film 222 disposed on a dielectric substrate. In this case, four terminals 223 to 226 are disposed on the ring conductive film 222 so as to define an angle of 90° relative to each other with respect to the center of the ring conductive film 222. Two of the four terminals arranged at the positions defining an angle of 90° relative to each other with respect to the center of the ring conductive film are connected to input-output coupling circuits 227 and 228, respectively. The remaining two terminals 225 and 226 are connected to each other via a feedback circuit 230.
Moreover, it is described that in the ring resonator including one strip line and having the above-described configuration, perpendicular resonance modes, which are not coupled to each other, are generated, and the coupling degree is controlled by the above-mentioned feedback circuit 230.
In the conventional dual mode band-pass filters shown in FIGS. 48 and 49, a two step band-pass filter can be formed by forming one conductive film pattern. Accordingly, the band-pass filter can be miniaturized.
However, the dual mode band-pass filters each have the configuration in which the input-output coupling circuits, which are separated from each other by a particular angle, are coupled to each other in the circular or square conductive film pattern. Therefore, the dual mode band-pass filters have the disadvantage that the coupling degree cannot be increased, and a wide pass band cannot be achieved.
In the band-pass filter shown in FIG. 48, the conductive film 201 is restricted to a circular shape. In the band-pass filter shown in FIG. 49, the conductive film 211 is also limited to a substantially square shape. Thus, there is the problem that the design flexibility is low.
Dual mode band-pass filters 221 using such a ring resonator as described in Japanese Unexamined Patent Application Publication Nos. 9-139612 and 9-162610 have the problem that it is difficult to improve the coupling degree, and the shape and size of the ring resonator are restricted.
On the other hand, in the dual mode band-pass filter 221 described in Japanese Unexamined Patent Application Publication No. 6-112701, the coupling degree is controlled, and the band-width can be widened by use of the feedback circuit 230. However, in the conventional dual mode filter, the feedback circuit 230 is required. Thus, the circuit configuration becomes complicated. Furthermore, the shape and size of the ring resonator are limited to a ring-shape, so that the design flexibility is very low.